The invention relates to a method for insulating stator windings for rotating electrical machines, in particular, direct current machines and alternating current machines.
In general, such electrical machines are provided with a stator and a rotor in order to convert mechanical energy into electrical energy (i.e., a generator) or, vice versa, to convert electrical energy into mechanical energy (i.e., an electric motor). Depending on the operating status of the electrical machine, voltages are generated in the conductors of the stator windings. This means that the conductors of the stator windings must be appropriately insulated in order to avoid a short circuit.
Stator windings in electrical machines can be constructed in different ways. It is possible to bundle several individual conductors that are insulated against one another and to provide the conductor bundle created in this manner, often called a conductor bar, with a so-called main insulation. To produce the stator windings, several conductor bars are connected with each other at their frontal faces. This connection can be made, for example, with a metal plate to which both the respective insulated individual conductors of the first conductor bar as well as the respective insulated individual conductors of the second conductor bar are connected in a conductive manner. The individual conductors of the conductor bar are therefore not insulated from each other in the area of the metal plate.
Alternatively to bundling the individual conductors into conductor bars, a long, insulated individual conductor is wound to a flat, oval coil that is called an original coil form, or xe2x80x9cfish.xe2x80x9d In a subsequent process, the so-called spreading, the original coil forms are transformed into their final shape and built into the stator.
With both of the above-described manufacturing techniques, both round and rectangular individual conductors can be used. The conductor bars or original coil forms produced from several individual conductors for the stator windings again may have round or rectangular cross-sections. The invention at hand preferably looks at conductor bars or original coil forms with a rectangular cross-section that were made from rectangular individual conductors. The conductor bars may be manufactured either as Roebel transpositions, i.e., with individual conductors twisted around each other, or not as Roebel transpositions, i.e., with untwisted individual conductors that extend parallel to each other.
According to the state of the art, mica paper that has been reinforced with a glass fabric carrier for mechanical reasons, is usually wrapped tape-like around the conductor in order to insulate the stator windings (e.g., conductor bars, original coil forms, coils). The wound conductor, which may also be shaped after being taped, is then impregnated with a hardening resin, resulting in a duroplastic, non-meltable insulation. Also known are mica-containing insulations with a thermoplastic matrix that are also applied to the conductor in the form of a tape, such as, for example, asphalt, shellac (Brown Boveri Review Vol. 57, p. 15: R. Schuler: xe2x80x9cInsulation Systems for High-Voltage Rotating Machinesxe2x80x9d), polysulfone and polyether ether ketone (DE 43 44044 A1). These insulations can be plastically reshaped when the melting temperature of the matrix is exceeded.
The insulations of stator windings that have been applied by wrapping have the disadvantage that their manufacture is time-and cost-intensive. In this context, special mention should be made of the wrapping process and impregnation process since they cannot be significantly accelerated any further because of the physical properties of the mica paper and impregnation resin. This manufacturing process is particularly prone to defects especially in the case of thick insulations, if the mica paper adapts insufficiently to the stator winding. In particular, an insufficient adjustment of the wrapping machine after wrapping the stator winding may result in wrinkles and tears in the mica paper, for example, because of a too steep or flat angle between the mica paper and the conductor, or because of an unsuitable static or dynamic tensile force acting on the mica paper during the wrapping. An excessive tape application may also result in overlaps that prevent uniform impregnation of the insulation in the impregnation tool. This may create a locally or generally defective insulation with reduced short-term or long-term stability. This significantly reduces the life span of such insulations for stator windings.
In addition, manufacturing processes for encasing conductor bundles are known from cable technology, whereby conductor bundles with a round cross-section are always encased with a thermoplast or with elastomers in an extrusion process. Document U.S. Pat. No. 5,650,031, which is related to the same subject matter as WO 97/11831, describes such a process for insulating stator windings in which the stator winding is passed through a central bore of an extruder. The stator winding, which has a complex shape, is hereby encased simultaneously with an extruded thermoplastic material at each side of the complex form, especially by coextrusion.
Also known from cable technology are polymeric insulations applied to the cables using a hot shrink-on technique. This relates to prefabricated sleeves with a round cross-section of curing thermoplasts, elastomers, polyvinylidene fluoride, PVC, silicone elastomer, or Teflon. After fabrication, these materials are stretched in their warm state and cooled. Once cooled, the material retains its stretched shape. This is accomplished, for example, because crystalline centers that fix the stretched macromolecules are formed. After repeated heating beyond the crystalline melting point, the crystalline zones are dissolved, whereby the macromolecules return to their unstretched state, and the insulation is in this way shrunk on. Also known are cold shrink-on sleeves that are mechanically stretched in their cold state. In the stretched state, these sleeves are pulled over a support structure that holds the sleeves permanently in the stretched state. Once the sleeves have been pushed and fixed over the components to be insulated, the support structure is removed in a suitable manner, for example, by pulling a spiral, perforated support structure out. But such shrink-on techniques cannot be used for stator windings with a rectangular cross-section since the sleeves with their round cross-section easily tear along the edges of the rectangular conductors, either immediately after shrinking or after strained briefly while the electrical machine is operated, because of the thermal and mechanical stresses.
Even while the stator windings are being manufactured, especially during the bending and handling of the conductors, particularly during installation into the stator, the insulation must be able to bear a significant high mechanical stress which could damage the insulation of the stator windings. The insulation of the stator winding conductors is also exposed to a combined stress during operation of the electrical machine. On the one hand, the insulation is dielectrically stressed between the conductor, to which is a high voltage is applied, and the stator, by a resulting electrical field. On the other hand, the heat generated in the conductor exposes the insulation to a thermal alternating stress, whereby a high temperature gradient is present in the insulation while the machine passes through the respective operating states. Because the materials involved expand differently, mechanical alternating stresses also occur. This results both in a shearing stress of the bond between conductor and insulation and a risk of abrasion at the interface between insulation and slot wall of the stator. Because of these high stresses, the insulation of the stator windings may tear, resulting in a short circuit. Consequently, the entire electrical machine will fail, and the repair will be time- and cost-intensive.
The invention involves a process for insulating stator windings for rotating electrical machines, whereby insulated stator windings are produced that ensure the insulation of the stator winding over the intended life span of the electrical machine.
The invention utilizes the fact that the elastomer is highly elastic, yet is able to withstand high thermal and electrical stresses. In the case of higher thermal stresses, silicone elastomer can be used advantageously.
Elastomers as a material for the main insulation promote the application of an injection molding process. The individual parts of the injection mold are preferably constructed in a modular manner for covering the conductor bar geometries that occur more frequently.
It is preferred that the conductor bars are centered with spacer elements or adjustable mandrels in the casting mold. The centering must be accomplished in such a way that the void between conductor bar and casting form has the same height at any point. The scope of this invention also includes providing main insulations with different thicknesses around the conductor bar. A uniform thickness of the main insulation is, however, a preferred embodiment.
In another method according to the invention, an internal corona shielding is applied between the insulating layer and the conductor surface. This is accomplished, for example, with a suitable injection molding process, in which several individual layers can be placed on top of each other.
In a particularly preferred method, the conductor bars are only brought into their final shape after being encased with the elastomer. The bending of the involutes greatly stretches the applied insulation. The use of elastomer according to the invention is hereby found to be particularly advantageous, since it reduces or even completely avoids the mechanical, electrical or thermal injury to the insulation that is being stressed by bending.